You can choose whether to show students marks as a letter grade (HD -> NN), a percentage, a raw mark or any combination of two.

From Grades > Setup > Categories and items click the 'Edit' next to the gradebook item and then 'Edit settings'. Under the 'Grade item' click 'Show more...' to bring up extra settings.

The default way grades are displayed for each activity in gradebook is as a letter grade (HD, DI, CR, PA, NN).

Use the 'Grade display type' dropdown to select how this activity's grades are displayed to students. Click the 'Save changes' button when finished.

Grades will be displayed this way both in gradebook and if the student looks for the grades in the activity directly. The exception is Turnitin where the numerical marks for each part of a Turnitin assignment will also be visible. If it is vital to hide the numerical mark and display only a letter grade in Turnitin, then use a manual gradebook item for displaying the Turnitin grade, and do not put the mark in Turnitin directly.